The problem of amplifying optical signals for long distance transmission was successfully addressed by the development of Erbium doped fiber amplifiers (EDFAs). An EDFA consists of a length of silica fiber with the core doped with ionized atoms (Er3+) of the rare earth element Erbium. The fiber is pumped with a laser at a wavelength of 980 nm or 1480 nm. The doped, pumped fiber is optically coupled with the transmission fiber so that the input signal is combined with the pump signal in the doped fiber. An isolator is generally needed at the input and/or output to prevent reflections that would convert the amplifier into a laser. Early EDFAs could provide 30 to 40 dB of gain in the C-band extending between 1530 to 1565 nm with noise figures of less than 5 dB. Recently, EDFAs have been developed that can provide similar performance in the L-band (1565 to 1625 nm) as well as in the C-band.
There is great interest in developing a broad or wide band amplifier that can amplify optical signals spanning the C- and L-bands and shorter wavelengths in the so-called “S-band” or “short-band”. Although poorly defined at present, the S-band is considered to cover wavelengths between about 1425 nm and about 1525 nm. Unfortunately, the gain in the S-band typically observed in EDFAs is limited by several factors, including incomplete inversion of the active erbium ions and by amplified spontaneous emissions (ASE) or lasing from the high gain peak near 1530 nm. Unfortunately, at present no efficient mechanism exists for suppressing ASE at 1530 nm and longer wavelengths in an EDFA.
The prior art offers various types of waveguides and fibers in which an EDFA can be produced. Most waveguides are designed to prevent injected light from coupling out via mechanisms such as evanescent wave out-coupling (tunneling), scattering, bending losses and leaky-mode losses. A general study of these mechanisms can be found in the literature such as L. G. Cohen et al., “Radiating Leaky-Mode Losses in Single-Mode Lightguides with Depressed-Index Claddings”, IEEE Journal of Quantum Electronics, Vol. QE-18, No. 10, October 1982, pp. 1467-72. U.S. Pat. Nos. 5,892,615 and 6,118,575 teach the use of W-profile fibers similar to those described by L. G. Cohen, or QC fibers to suppress unwanted frequencies and thus achieve higher output power in a cladding pumped laser. Such fibers naturally leak light at long wavelengths, as discussed above, and are more sensitive to bending than other fibers.
In producing an EDFA for the S-band the relatively high losses and low gains over the S-band render the selection of fiber and fiber profile even more difficult. In fact, the problems are so severe that the prior art teaches interposition of external filters between EDFA sections to produce an S-band EDFA. For example, Ishikawa et al. disclose a method of fabricating an S-band EDFA by cascading five stages of silica-based EDFA and four ASE suppressing filters in Ishikawa et al., “Novel 1500 nm-Band EDFA with discrete Raman Amplifier”, ECOC-2001, Post Deadline Paper. In Ishikawa et al.'s experimental setup, the length of each EDA is 4.5 meters. The absorption of each suppressing filter at 1.53 μm is about 30 dB and the insertion losses of each suppressing filter at 1.48 μm and 0.98 μm are about 2 dB and 1 dB respectively. The pumping configuration is bi-directional, using a 0.98 μm wavelength to keep a high inversion of more than D≧0.7 (D, relative inversion). The forward and backward pumping powers are the same and the total pumping power is 480 mW. Ishikawa et al. show a maximum gain of 25 dB at 1518.7 nm with 9 dB gain tilt.
This method is relatively complicated and not cost-effective, as it requires five EDFAs, four ASE suppressing filters and high pump power. Also, each of the ASE suppressing filters used in Ishikawa et al.'s method introduces an additional insertion loss of 1-2 dB. The total additional insertion loss is thus about 4-8 dB.
In U.S. Pat. No. 6,049,417 Srivastava et al. teach a wide band optical amplifier which employs a split-band architecture. This amplifier splits an optical signal into several independent sub-bands that then pass in parallel through separate branches of the optical amplifier. Each branch may be optimized for the sub-band that traverses it. In one embodiment Srivastava et al. teach to equip one of the branches with a number of S-band EDFAs and a number of gain equalization filters (GEFs) interposed between the S-band EDFAs to obtain amplification in the S-band. The other sub-bands in this embodiment have EDFAs for amplifying the C- and L-bands respectively. Unfortunately, the S-band branch of this wide band amplifier suffers from similar disadvantages as discussed above in conjunction with Ishikawa.
In view of the above, it would be an advance in the art to provide a wide band amplifier that amplifies optical signals spanning the S-, C- and L-bands and exhibits high efficiency in the S-band. Specifically, it would be an advance to provide such wide band amplifier that amplifies optical signals in the S-band without requiring many filters and takes full advantage of a minimum number of pump sources.